Drop on demand inkjet printing systems eject ink drops from printhead nozzles in response to pressure pulses generated within the printhead by either piezoelectric devices or thermal transducers, such as resistors. The ink drops are ejected toward an image receiving surface where each ink drop forms a pixel of an ink image on the image receiving surface. The printheads have a plurality of inkjet ejectors that are fluidly connected at one end to an ink supplying manifold through an ink channel and at another end to an aperture in a face plate of the printhead.
In some phase change or solid ink printers, known as direct printers, the printer ejects ink drops directly onto a print medium such as a paper sheet. After ink drops are printed on the print medium, the printer moves the print medium through a nip formed between two rollers that apply pressure and optionally heat to the ink drops and print medium. One roller, referred to as the “spreader roller” contacts the printed side of the print medium. The spreader roller is heated and coated with a release agent that prevents ink drops on the print medium from transferring onto the spreader roller. The second roller is referred to as a “pressure roller.” This roller presses the media against the spreader roller. The pressure roller may be optionally heated to facilitate the fixing of the ink to the sheet of print medium. The heat and pressure applied through the nip flattens the ink drops and secures the printed ink image to the print medium in a process known as “fixing.”
In an indirect printing embodiment, the printheads eject ink drops onto the surface of an intermediate image receiving member such as a rotating drum or endless belt. A “transfix” roller is positioned against the intermediate image receiving member to form a transfix nip. As a media sheet passes through the transfix nip in synchronization with the ink image on the intermediate image receiving member, the ink image transfers and fixes to the media sheet under pressure and heat in the transfix nip. The transfer and fixation of the ink image are well known to the art and are referred to as a transfix process.
Both direct and indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print medium. Duplex printing produces an image on each side of a media sheet. In duplex direct printing, an ink image is formed on a first side of the media sheet, which then passes through the spreader nip to fix the ink image onto the first side of the media sheet. The medium is then inverted and sent along a path that passes the second side of the media sheet by the printheads for the formation of a second ink image on the second side. The sheet then returns to the spreader nip where the second ink image is fixed to the second side of the media sheet. A similar process is used with indirect printing, except the image is initially formed on an intermediate drum and then transferred to the media and fixed in the nip at the same time.
In both direct and indirect printing systems, having significant levels of oil on the media before imaging is undesirable, as the release agent can prevent ink from properly adhering or transferring to the media. Therefore, in a duplex printing process, preventing the release agent from transferring to the back side of a sheet during printing of the first side image is desirable. To achieve this goal, current printing systems slow down the transfix process and use special sheet and nip formation sequencing during duplex printing to prevent release agent from being transferred to the back of a sheet during front side printing. One technique for minimizing this problem is synchronizing the transfix or pressure rollers with the media sequencing so that the portion of the roller that contacts the back of the media sheet only contacted another media sheet on the previous revolution. The portion contacted was thus not in direct contact with the intermediate drum or the spreader roller, which would have transferred excess oil to the transfix or pressure roller surface and thus to the back of the present sheet. Unfortunately, synchronization of the rollers may not prevent release agent from transferring to media sheets having non-uniform edges, such as media sheets having extended tabs, pre-punched holes, or different sizes. Consequently, improved operation of direct and indirect printers that addresses the limited ability of current printers to keep release agent from tabbed, hole punched, and other non-uniform sized media sheets would be beneficial to higher throughput and image quality in such printers.